A. Field of the Invention
The present invention relates to purification of contaminated liquids and effluent, and more particularly to apparatus and methods for the production and utilization of submicron gas bubbles to facilitate wastewater treatment, biopurification processes and desalination pretreatment.
B. Description of the Related Art
Population growth and the worldwide expansion of heavy industry have dramatically increased the amount of wastewater and other deleterious fluid effluent introduced into the environment. The need to cope with ever-growing volumes of contaminated liquids has engendered significant and varied research into ways of separating hazardous or polluting materials from a bulk liquid source. A growing number of fluid-treatment methodologies rely on controlled introduction of gas into the contaminated fluid.
In one approach, frequently termed "bioremediation," microorganisms digest waste products dissolved or entrained in a contaminated liquid and, in so doing, convert them into inert or less harmful substances. These microorganisms typically require large available quantities of oxygen to function effectively. In a typical bioremediation system, waste-degrading microorganisms are contained in a bioreactor equipped with a recycle loop, where the circulating liquid is aerated to nourish the microorganisms. For example, U.S. Pat. No. 5,151,187 teaches use of a membrane filtration system in tandem with an in-line membrane micronizer along the recycle loop. The micronizer includes a tubular porous membrane surrounded by a hollow chamber. Fluid passes axially through the bore of the membrane while air, introduced into the housing under pressure, penetrates the pores radially to form small bubbles along the inner wall of the membrane; these bubbles enter the flowing liquid, and feed the microorganisms.
"Flotation" systems utilize gas bubbles mechanically to draw particulate matter to the surface of the bulk liquid, where it is readily skimmed off and disposed of. Naturally, the effectiveness of such systems depends both on the capacity of the bubbles to transport particles within the liquid and a sufficient number of bubbles to ensure rapid interaction with and capture of particles.
Other gasification systems, such as chlorination or ozonation of drinking water, rely on the bactericidal or other direct effects of the gas on suspended or dissolved contaminants.
In all of these systems it is the interfacial contact area, rather than the absolute volume of introduced gas that determines its utilization--by microorganisms, or in terms of ability to sequester or otherwise interact with contaminant--and researchers have therefore recognized the benefits of introducing gas in the form of finely sized bubbles to obtain the maximum possible gas surface area. In one well-known approach, gas is dissolved in a liquid at pressures above atmospheric in accordance with Henry's Law, and environmental conditions (typically temperature and/or pressure) subsequently altered to reduce gas solubility. The change in conditions forces dissolved gas out of solution, at nucleation sites, in the form of small bubbles. Proper control of the amount of dissolved gas, the number of available nucleation sites, and the final conditions determines mean bubble size, with typical diameters in the range of 50-100 microns.
A primary disadvantage of this approach stems from practical operating constraints that limit the amount of dissolved gas (and therefore the overall generation rate and size of bubbles) in a liquid. For example, gas solubility is negatively affected by increasing temperature, necessitating relatively low operating temperatures during the initial dissolution phase. This may not be practical in large operating environments. Moreover, after dissolution, significant input of energy and time may be required to obtain the higher temperatures needed to selectively reduce gas solubility and form the small bubbles.
A second approach is to mechanically shear larger bubbles to reduce their mean diameter. Typical methods include feeding air to the suction of a centrifugal pump, entrapment by an aspirator, and entrainment by surface injection through the gas/liquid interface. These systems frequently require large quantities of energy for proper operation, and may be prone to various design complications (such as impeller cavitation). Furthermore, mechanical reduction of bubble size typically yields bubbles of mean diameter 200-5000 microns. Physical laws controlling impingement largely foreclose the possibility of creating bubbles smaller than 100 microns, and energy considerations render bubble sizes smaller than 200-300 microns impractical.
A third approach, as described in the '187 patent and U.S. Pat. No. 5,122,312, involves use of porous media (of tubular or plate configuration) to inject gases from a flat or convex surface. Such devices are subject to "fouling" or clogging of the pores by the wastewater (or particulate matter entrained therein) as it passes over the pores.
In addition to specific design disadvantages associated with the various methods of obtaining small bubbles, certain physical limitations tend to restrict the mean useful bubble diameter regardless of how the bubbles are generated. First, bubbles tend to coalesce into larger gas pockets if brought or allowed to remain too close to one another. For example, porous-media systems tend to exhibit significant coalescence at high bubble-generation rates, as bubbles crowd one another exiting the surface into the surrounding liquid. High surface porosity, generally used to reduce the energy requirements and material cost of the porous elements, aggravates this condition. Coalescence is further increased by the quiescent boundary layer (which is thick relative to bubble diameter) at the surface of the porous element, which retains the bubbles in close proximity to one another as they are formed. Conventional attempts to reduce the thickness of the boundary layer by increasing flow velocity ultimately fail to reduce coalescence, since they increase contact among bubbles.
A second limitation occurs as a result of pressure drop. Because of the inverse relationship between gas volume and pressure at constant temperature, bubble diameter increases with diminishing pressure. Thus, in the case of porous-media systems, high rates of bubble generation typically require long stretches of porous media, with pressure dropping lengthwise from inlet to outlet. The result is a net increase in bubble size at the outlet relative to the size at formation, as well as a wide range of bubble sizes due to the varying pressure drops experienced by bubbles formed at different points along the medium's length.